Vacuum system, in particular EUV lithography system, and optical element

ABSTRACT

A vacuum system, in particular an EUV lithography system, includes: a vacuum housing, in which a vacuum environment is formed, and also at least one component (14), e.g., an optical element, having a surface (14a) which is subjected to contaminating particles in the vacuum environment. A surface structure (18) is formed at the surface in order to reduce adhesion of the contaminating particles, said surface structure having pore-shaped depressions (24) separated from one another by webs (25). The optical element has a substrate (19), and a multilayer coating (20) applied to the substrate and configured to reflect EUV radiation (6). The surface structure formed at the surface (14a) of the multilayer coating (20) reduces adhesion of contaminating particles (17) via pore-shaped depressions (24) separated from one another by webs (25).

CROSS REFERENCE TO RELATED APPLICATIONS

This is a Continuation of International Application PCT/EP2015/068066,which has an international filing date of Aug. 5, 2015, and thedisclosure of which is incorporated in its entirety into the presentContinuation by reference. The following disclosure is also based on andclaims the benefit of and priority under 35 U.S.C. § 119(a) to GermanPatent Application No. DE 10 2014 216 118.5, filed Aug. 13, 2014, whichis also incorporated in its entirety into the present Continuation byreference.

FIELD OF THE INVENTION

The invention relates to a vacuum system, in particular an EUVlithography system, comprising: a vacuum housing, in which a vacuumenvironment is formed, and also at least one component having a surfacewhich is subjected to contaminating particles in the vacuum environment.The invention also relates to an optical element, comprising: asubstrate, and also a multilayer coating for reflecting EUV radiation,said multilayer coating being applied to the substrate.

BACKGROUND

In specific optical arrangements, for example in extreme ultraviolet(EUV) lithography systems, it is necessary to arrange at least oneportion of the beam path and thus also at least one portion of theoptical elements in a vacuum environment. Such a vacuum environment cantypically have a (total) pressure of between approximately 10⁻⁹ mbar andapproximately 10⁻¹ mbar in EUV lithography systems.

Within the meaning of this application, an EUV lithography system isunderstood to be an optical system for EUV lithography, i.e. an opticalsystem which can be used in the field of EUV lithography. Alongside anEUV lithography apparatus that serves for producing semiconductorcomponents, the optical system can be, for example, an inspection systemfor inspecting a photomask (also called reticle hereinafter) used in anEUV lithography apparatus, or for inspecting a semiconductor substrate(also called wafer hereinafter) to be structured, or a metrology systemthat is used for measuring an EUV lithography apparatus or partsthereof, for example for measuring a projection system.

In an EUV lithography system, but also in other optical arrangements,the presence of contaminating substances or particles in the vacuumenvironment or in the residual gas atmosphere present therein cannot becompletely avoided. The contaminating substances can be polymers, forexample, which originate from vacuum pumps or which are outgassed fromadhesives. The contaminating substances can also be residues ofphotoresists applied on the wafer which are outgassed from thephotoresist under the influence of the operating radiation and which canlead to carbon contaminations on the optical elements of the EUVlithography system or on other components in the vacuum environment.

It is known to remove contaminating substances or particles from opticalsurfaces with the aid of one gas nozzle or a plurality of gas nozzles,as is described for example in WO 2009/059614 A1 in the name of theapplicant. For this purpose, the gas nozzle is aligned with the surfaceto be cleaned, and the surface to be cleaned is brought into contactwith a gas flow of a cleaning gas, e.g. in the form of activatedhydrogen or in the form of hydrogen radicals.

The efficiency of such cleaning depends on how strongly thecontaminating particles adhere to the surface to be cleaned, i.e. thestrength of the adhesion of the particles to the surface. Generally, ina vacuum environment, surfaces should be avoided which foster particledeposits, in particular surfaces having a high roughness, for whichreason components having surfaces composed of electropolished high-gradesteel or aluminium having very low roughness are often used in vacuumenvironments.

DE 10 2009 044 462 A1 discloses an optical element for filteringelectromagnetic radiation, said optical element having a multilayerstructure designed for reflecting EUV radiation. The optical elementalso has a grating structure designed for diffracting radiation in thevisible to infrared wavelength range. In one example, the gratingstructure is designed for the destructive interference of radiationhaving an infrared wavelength of e.g. 10.6 μm. On the grating structure,it is possible to arrange an additional grating structure having asmaller grating constant and depth, which generates a destructiveinterference of radiation at least one further wavelength that issignificantly shorter than the wavelength of the radiation that isfiltered by the grating structure.

SUMMARY

It is an object of the invention to provide a vacuum system, inparticular an EUV lithography system, and also an optical element whichfeature reduced adhesion of particles on at least one surface.

This and other objects are achieved with a vacuum system of the typementioned in the introduction in which a surface structure is formed atthe surface which is arranged in the vacuum environment and thereforecomes into contact with contaminating particles, in order to reduce theadhesion of the contaminating particles, said surface structure havingpore-shaped depressions separated from one another by webs.

The inventor has recognized that a surface structure having pore-shapeddepressions, i.e. blind holes typically having a small depth of theorder of magnitude of micrometers or, if appropriate, nanometers, canexhibit a more greatly reduced adhesion than is the case for acompletely smooth surface. The pore-shaped depressions typically are notconnected to one another since they are separated from one another bythe webs and are typically distributed in a substantially regulararrangement over the surface. The pore-shaped depressions generally havea substantially rectangular depth profile. In particular, the lateraledges of the pore-shaped depressions should be as steep as possible.

For reducing the adhesion, use is made of the fact that particles whichcome into contact with a surface experience an adhesion force that issubstantially based on the interaction within the area of contact withthe surface. The area of contact can be described for example by thecontact radius in the so-called JKR model (K. L. Johnson, K. Kendall, A.D. Roberts, “Surface energy and the contact of the elastic solids”,Proc. Roy. Soc. London 324, 301 (1971)). For particles which are indirect contact with the surface, the interaction of the particles withthe surface is dominated by van der Waals forces, cf. L. Gradon,“Resuspension of particles from surfaces: Technological, environmentaland pharmaceutical aspects”, Adv. Powder Tech. 20, 17 (2009). On accountof the short range of the van der Waals forces, a small distance betweenthe particles and the surface already results in a significant reductionof the adhesion forces. For the energy U_(vdw) between two interactingbodies as a function of the distance d, the following relationshipgenerally holds true here:U _(vdw) =−B/d ⁶,  (1)wherein B denotes the interaction coefficient.

To describe the adhesion of a particle of arbitrary shape, forillustrative reasons recourse can also be made to the concept of thesummation of the pairwise interactions (according to Hamaker), cf. H. C.Hamaker, “The London-van der Waals attraction between sphericalparticles”, Physica IV, 10, 1058 (1937). A particle (in the same way asthe associated surface) can be described as a finite number of entities,wherein the contribution to the total adsorption of each entity of theparticle arises as a result of summation of the interaction energiesover all the entities of the surface under consideration in accordancewith equation (1). Although this approach disregards matrix effects thatoccur, it is a good approximation for the case of small distances d, cf.E. M. Lifshitz, “The theory of molecular attractive forces betweensolids”, Soviet Phys. JETP, 2, 73 (1956) and can thus be useful for theargumentation below.

In one advantageous embodiment, the pore-shaped depressions have adiameter which is smaller than the diameter of the contaminatingparticles in the vacuum environment whose adhesion to the surface isintended to be reduced. Within the meaning of this application, thediameter of a particle is understood to be that diameter of a spherewhose volume corresponds to the volume of the (generally non-spherical)particle. The diameter of the pore-shaped depression is understood to bethat diameter of a circle whose surface corresponds to the surface ofthe (not necessarily circular) pore-shaped depression. In this case, thesurface of the pore-shaped depression is measured at the top sidethereof facing the vacuum environment.

The principle of the surface structure for reducing particle adhesion asdescribed in the present application is based on the sum of all thepairwise interactions being significantly reduced by virtue of the factthat the possible number of near-surface atoms within a particle,represented by the area of contact between particle and surface, isgreatly reduced on account of steric hindrance. For particles having aspecific size or order of magnitude, this can be achieved by the surfacebeing provided with a substantially regular surface structure over thewhole area, in the case of which surface structure the diameters of thepore-shaped depressions are smaller than the diameters of a respectivecontaminating particle whose adhesion to the surface is intended to beprevented.

On account of their larger diameter, the particles cannot penetrate intothe pore-shaped depressions and therefore rest on the circumferentialedges of the webs which form the area of contact. Since pore-shapeddepressions having a given diameter generally cannot effectively preventthe adhesion of particles having a significantly larger particlediameter, it can be advantageous to form a surface structure havingpore-shaped depressions having different diameters, in particular asurface structure having pore-shaped depressions having diameters ofdifferent orders of magnitude. In this case, the diameter specifiedabove constitutes a minimum diameter of the pore-shaped depressions,which constitutes a lower limit for the particle size which can beprevented from adhesion to the surface with the aid of the surfacestructure.

The width of the webs should not be chosen to be excessively large, inorder to prevent the particles from resting on the top side of the webs.This can be achieved, for example, by the webs having a width that islikewise smaller than the diameter of the contaminating particles. Thepore-shaped depressions can be arranged at the surface of the componentfor example in a regular, in particular hexagonal, pattern.

In one embodiment, the pore-shaped depressions have a diameter of lessthan e.g. 10 nm. As was described further above, the diameter of thedepressions defines the minimum particle size or the minimum particlediameter which can be prevented from adhesion with the aid of thesurface structure.

Preferably, the web widths of the surface structure are smaller than thediameters of the pore-shaped depressions of the surface structure. Theconstitution of the webs should be chosen such that the adhesion ofparticles to the top side of the webs is of the order of magnitude ofthe adhesion of particles which are situated above the pore-shapeddepressions. This can be achieved by the choice of a suitable ratiobetween the diameter of the pore-shaped depressions and the width of thewebs, wherein the width of the webs, as specified above, generallyshould not be greater than the diameter of the pore-shaped depression.

In a further embodiment, the depth of a respective pore-shapeddepression is at least as large as half the diameter of a respectivepore-shaped depression. The pore-shaped depressions should not have anexcessively small depth, in order to prevent the particles that rest onthe edges of the webs from reaching the bottom of the depression and thearea of contact between the particles and the surface being increased asa result.

In a further embodiment, the surface structure has at least one periodicpore structure. A periodic or quasi-periodic pore structure isunderstood to be a structure in which the pore-shaped depressions (ofthe same size) and the webs (of the same size) form a regular pattern.The periodic pore structure need not necessarily be exactly periodic,rather it suffices if a substantially periodic pore structure is presentin which the diameters of different pore-shaped depressions and thewidths of different webs are substantially of the same magnitude, as isthe case e.g. for a self-structuring. The period length of the porestructure is defined as the sum of the diameter of a pore-shapeddepression and the width of a web between two adjacent pore-shapeddepressions. The effect of such a pore structure with regard to thereduction of the adhesion of particles is efficient if the periodicityof the pore structure is of the order of magnitude of the particlediameter. As a result of a pore structure being provided, therefore, theadhesion of particles of a specific order of magnitude can be preventedfrom adhesion.

Preferably, the periodic pore structure has a period length of less thane.g. 10 nm. The period length of the pore structure defines the minimumparticle diameter which can be prevented from adhesion by the porestructure. As was described further above, it is necessary for thediameter of a pore-shaped depression to be smaller than the diameter ofthe particle to be prevented from adhesion. The width of the webs shouldtypically not be greater than the diameters of the depressions.

In one development, the surface structure has a first periodic porestructure having a first period length and a second periodic porestructure applied to the first periodic pore structure and having asecond period length, which is smaller than the period length of thefirst periodic pore structure. As was described further above, theperiod length defines the minimum particle diameter or the order ofmagnitude of the diameter of the particles whose adhesion to the surfacecan be prevented. Particles having a particle diameter that issignificantly greater than the period length of the pore structurecannot be effectively prevented from adhesion by the pore structure.Therefore, it is advantageous to produce a surface structure having twoor more pore structures having different period lengths, which preventsparticles having different orders of magnitude of the particle diameterfrom adhesion.

The surface structure can have a third periodic pore structure—appliedto the second periodic pore structure—having a third period length,which is smaller than the period length of the second periodic porestructure. Correspondingly, the surface structure can also have afourth, fifth, etc. pore structure having a respectively decreasingperiod length in order to prevent particles having a plurality ofdifferent orders of magnitude of the particle diameter from adhesion.

In one development, the first period length P1 is at least five timesthe magnitude of the second period length P2, that is to say thatP1>5×P2 holds true. The period lengths of the pore structures should notbe too close together, in order to prevent particles having differentorders of magnitude of the particle diameter from adhesion. Inparticular, the second period length can also be at least five times themagnitude of the third period length of a third pore structure possiblypresent.

A surface structure having the properties described above can beproduced in various ways. By way of example, the surface can bestructured using a photolithographic process. In order to realize alarge-area structuring of a surface with comparatively little effort, itis possible to have recourse to structuring methods which enable acertain degree of self-assembly.

In the case of a component having a surface composed of aluminium,highly ordered aluminium oxide layers having hexagonally arranged porescan be produced by anodic oxidation in aqueous electrolytes undersuitable conditions (pH, electrolyte, voltage and temperature). In thiscase, e.g. by varying the voltage, it is possible to define the porediameter and thereby to realize periodicities of from a few nanometersto the micrometers range, as is described in the article by A. P. Li etal. “Hexagonal pore arrays with a 50-420 nm interpore distance formed byself-organisation in anodic alumina”, J. Appl. Phys. 84 (11), 6023(1998).

For very small periodicities of a few nanometers, the structures canalso be realized through a micellar approach, which is based on theself-assembly of block copolymers loaded with metal salts and asubsequent lithography process, as is described in the article by B.Gorzolnik et al. “Nano-structured micropatterns by combination of blockcopolymer self-assembly and UV photolithography”, Nanotechnology 17,5027 (2006).

In order to produce the superimposition of a plurality of periodic porestructures having different period lengths, as described further above,it is possible to use a multiple structuring process in which porestructures having a large pore diameter or a large period length arefirstly produced in a first structuring step and pore structures havinga smaller period length are produced in at least one subsequentstructuring step.

A further aspect of the invention relates to an optical element of thetype mentioned in the introduction in which a surface structure isformed at the surface of the multilayer coating in order to reduce theadhesion of contaminating particles. The surface structure haspore-shaped depressions separated from one another by webs. In thisaspect of the invention, the component which is subjected to thecontaminating particles is an optical element of EUV lithography, whichcan be used in particular in the EUV lithography system describedfurther above. The depth of the surface structure should not be chosento be excessively large, in order to prevent the reflectivity of themultilayer coating for the EUV radiation from being impaired to anexcessively great extent. In this case, the depth of the pore-shapeddepressions should typically not be greater than approximately 3 μm. Notonly components in the form of optical elements but also othercomponents, in particular vacuum components, e.g. vacuum housings, orhousings of sensors which are arranged in the vacuum environment, can beprovided with the surface structure described further above.

In one embodiment, the pore-shaped depressions have a diameter which issmaller than the diameter of the contaminating particles whose adhesionto the surface is intended to be reduced. As was described furtherabove, the diameter of the depressions determines the minimum diameterof particles which can be prevented from adhesion with the aid of thesurface structure.

In a further embodiment, the or all pore-shaped depressions at thesurface have a diameter of less than e.g. 10 nm. As was described above,the minimum diameter of the pore-shaped depressions defines the smallestparticle diameter of the particles which can be prevented from adhesionby the surface structure.

In one embodiment, the web widths of the surface structure are smallerthan the diameters of the pore-shaped depressions of the surfacestructure, such that no large-area interaction between particle andsurface can occur. As an alternative to defining this ratio by choosingthe diameter of the pore-shaped depressions with respect to the width ofthe webs, it is also possible for the desired ratio to be established asa side effect of the superimposition of a plurality of periodicstructures having different period lengths, as described further above,since, during the application of a pore structure having a smallerperiod length, the webs of the underlying pore structure having a largerperiod length are structured as well.

In a further embodiment, the depth of a respective pore-shapeddepression is at least as large as half the diameter of a respectivepore-shaped depression. An undesirable contact between the particles andthe bottom of the pore-shaped depressions can be avoided in this way.

In a further embodiment, the surface structure has at least one periodicpore structure which preferably has a period length of less than e.g. 10nm. Such a periodic pore structure can be produced by aself-structuring, for example.

In one development, the surface structure has a first periodic porestructure having a first period length and a second periodic porestructure applied to the first periodic pore structure and having asecond period length, which is smaller than the period length of thefirst periodic pore structure. As was described further above, in thisway particles having particle diameter having different orders ofmagnitude can be prevented from adhesion. In order to prevent particleshaving different orders of magnitude of the particle diameter fromadhering to the surface, preferably the first period length is at leastfive times the magnitude of the second period length.

Further features and advantages of the invention are evident from thefollowing description of exemplary embodiments of the invention, withreference to the figures of the drawing which show details essential tothe invention, and from the claims. The individual features can each berealized individually by themselves or as a plurality in any desiredcombination in a variant of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments are illustrated in the schematic drawing and areexplained in the following description. In the figures:

FIG. 1 shows a schematic illustration of an EUV lithography apparatus,

FIG. 2 shows a schematic illustration of a particle on a surface,

FIGS. 3A and 3B show schematic illustrations of a surface structureformed on a multilayer coating of an EUV mirror, in a plan view and in asectional illustration, and

FIG. 4 shows a schematic illustration of a surface structure havingthree periodic pore structures having different period lengths.

In the following description of the drawings, identical reference signsare used for identical or functionally identical components.

DETAILED DESCRIPTION

FIG. 1 schematically shows a vacuum system in the form of an EUVlithography apparatus 1 consisting of a beam shaping system 2, anillumination system 3 and a projection system 4, which are accommodatedin separate vacuum housings (designated with the same reference signs)and are arranged successively in a beam path 6 proceeding from an EUVlight source 5 of the beam shaping system 2. By way of example, a plasmasource or a synchrotron can serve as EUV light source 5. The emergingradiation in the wavelength range of between approximately 5 nm andapproximately 20 nm is firstly focussed in a collimator 7. With the aidof a downstream monochromator 8, the desired operating wavelength isfiltered out by varying the angle of incidence, as indicated by adouble-headed arrow. In the stated wavelength range, the collimator 7and the monochromator 8 are usually embodied as reflective opticalelements, wherein at least the monochromator 8 has no multilayer coatingat its optical surface, in order to reflect a wavelength range havingthe highest possible bandwidth.

The radiation treated with regard to wavelength and spatial distributionin the beam shaping system 2 is introduced into the illumination system3, which has a first and a second reflective optical element 9, 10. Thetwo reflective optical elements 9, 10 direct the radiation onto aphotomask 11, which operates as a further reflective optical element,said photomask having a structure which is imaged onto a wafer 12 on areduced scale via the projection system 4. For this purpose, a third anda fourth reflective optical element 13, 14 are provided in theprojection system 4.

The reflective optical elements 9, 10, 11, 12, 13, 14 respectively havean optical surface 9 a, 10 a, 11 a, 12 a, 13 a, 14 a, which are arrangedin the beam path 6 of the EUV lithography apparatus 1. A further,mechanical component 15 is also arranged in the projection system 4, forexample in the form of a sensor or of part or, if appropriate, of theentire inner side 2 a of a housing wall of the vacuum housing 2 (or aninner surface 3 a, 4 a of a housing wall of the other vacuum housings 3,4). The component 15 likewise has a surface 15 a arranged in a vacuumenvironment 16 in the projection system 4. The vacuum environment 16 isgenerated with the aid of vacuum pumps (not shown). The total pressurein the vacuum environment 16 of the beam shaping system 2, of theillumination system 3 and of the projection system 4 can be different.The total pressure is typically in the range of between approximately10⁻⁹ mbar and approximately 10⁻¹ mbar.

As can likewise be seen in FIG. 1, the vacuum environment 16 of theprojection system 4 contains contaminating particles 17 to which thesurfaces 9 a to 14 a of the optical elements 9 to 14 and the surface 15a of the mechanical component 15 are subjected. FIG. 2 shows by way ofexample a detail from the surface 15 a of the mechanical component 15,which is a vacuum component composed of aluminium in the example shown.A particle 17 illustrated as spherical in an idealized way has depositedon the planar, polished surface 15 a of the component 15. The area ofcontact, assumed to be circular likewise in an idealized way, betweenthe particle 17 and the surface 15 a has a contact radius r_(k) that isrelatively large in comparison with the diameter d_(p) of the particle17.

In order to reduce the area of contact between the particle 17 and thesurface 15 a, a surface structure 18 can be applied to the surface 15 a,said surface structure reducing the area of contact of the particle 17with the surface 15 a and thus the adhesion of the particle 17 to thesurface 15 a. Such a surface structure 18, which can also be provided onthe surface 15 a of the component 15, is shown in FIGS. 3A and 3B on thebasis of the example of the last optical element 14 in the beam path ofthe EUV lithography apparatus 1 from FIG. 1.

The optical element 14 shown in a sectional view in FIG. 3B comprises asubstrate 19 and a multilayer coating 20 applied to the substrate 19.The multilayer coating 20 comprises alternately applied layers of amaterial having a higher real part of the refractive index at theoperating wavelength λ_(B) (also called spacers 21) and of a materialhaving a lower real part of the refractive index at the operatingwavelength λ_(B) (also called absorbers 22), wherein an absorber-spacerpair forms a stack. This construction of the multilayer coating 20simulates in a way a crystal whose lattice planes correspond to theabsorber layers at which Bragg reflection takes place. The thicknessesof the individual layers 21, 22 and of the repeating stacks can beconstant or else vary over the entire multilayer coating 20, dependingon what spectral or angle-dependent reflection profile is intended to beachieved. The absorber and spacer materials can have constant or elsevarying thicknesses over all the stacks in order to optimize thereflectivity. Furthermore, it is also possible to provide additionallayers for example as diffusion barriers between spacer and absorberlayers 21, 22.

In the present example, in which the optical element 14 was optimizedfor an operating wavelength λ_(B) of 13.5 nm, i.e. in the case of anoptical element 14 which has the maximum reflectivity for substantiallynormal incidence of radiation at a wavelength of 13.5 nm, the stacks ofthe multilayer coating 20 have alternate silicon and molybdenum layers.In this case, the silicon layers correspond to the layers 21 having ahigher real part of the refractive index at 13.5 nm, and the molybdenumlayers correspond to the layers 22 having a lower real part of therefractive index at 13.5 nm. Other material combinations such as e.g.molybdenum and beryllium, ruthenium and beryllium or lanthanum and B₄Care likewise possible, depending on the operating wavelength.

As shown in FIG. 3B, a surface structure 18 is formed at the surface 14a of the multilayer coating 20, said surface structure having a periodicpore structure 23 having pore-shaped depressions 24 separated from oneanother by webs 25, wherein the surface structure 18 has a substantiallyhexagonal structure, as illustrated in FIG. 3A (each pore-shapeddepression 24 is surrounded by six further pore-shaped depressions 24).The pore-shaped depressions 24 have a substantially circular geometryhaving a diameter d_(V) which, in the example shown, is smaller than thediameter d_(P) of a particle 17 arranged above the pore-shapeddepression. In the sectional view through the optical element 14 asshown in FIG. 3b , the pore-shaped depressions 24 and the webs 25 form abinary or rectangular surface profile, i.e. the flanks of the webs 25run approximately vertically and the bottom of the pore-shapeddepressions 24 runs substantially parallel to the planar surface of thesubstrate 19 to which the multilayer coating 20 is applied.

As shown in FIG. 3B, the period length d_(s) of the periodic porestructure 23, which corresponds to the sum of the diameter d_(V) of thepore-shaped depression 24 and the width B of the web 25, is slightlylarger than the diameter d_(P) of the particle 17. By contrast, thediameter d_(v) of the pore-shaped depression 24 is slightly smaller thanthe diameter d_(P) of the contaminating particle 17. The area of contactbetween the contaminating particle 17 and the surface 14 a of theoptical element 14 is therefore formed exclusively by the circularlycircumferential edge 25 a of the web 25, which is significantly smallerthan the area of contact between the particle 17 and the planar surface14 a from FIG. 2.

In the example shown, the depth T of a respective pore-shaped depression24 is somewhat more than half the magnitude of the diameter d_(P) of thepore-shaped depression 24. In this way, it is ensured that a sphericalparticle 17 that is slightly larger than the diameter d_(P) of thepore-shaped depression 24, if it contacts the circumferential edge 25 aof the web 25 delimiting the depression 24, does not rest on the bottomof the depression 24 and the area of contact with the surface 14 a isincreased in this way.

In general, the adhesion of particles 17 that are situated above thepore-shaped depressions 24 should be of the order of magnitude of theadhesion of particles 17 that are situated on the webs 25. The ratiobetween the adhesion at the depressions 24 and the adhesion at the webs25 can be set by the ratio between the diameter d_(v) of the pore-shapeddepressions 24 and the width B of the webs 25. In principle, in the caseof a surface structure 18 having exactly one periodic pore structure 23,it has proved to be advantageous if the widths B of the webs 25 of thesurface structure 18 are smaller than the diameters d_(V) of thepore-shaped depressions 24 of the surface structure 18. Fulfilling sucha condition imposed on the widths B of the webs 25 is generally notnecessary, however, if the surface structure 18 has two or more, forexample three, periodic pore structures 23 a-c, as is shown in FIG. 4.

The surface structure 18 shown in FIG. 4 has a first periodic porestructure 23 a, which has a first period length d_(S1) and which servesfor reducing the adhesion of particles 17 a having a first (average)particle diameter d_(P1). A second periodic pore structure 23 b issuperimposed on the first periodic pore structure 23 a, said secondperiodic pore structure having a second, smaller period length d_(S2)and serving for reducing the adhesion of particles 17 b having a second,smaller particle diameter d_(P2). A third periodic pore structure 17 cis superimposed on the second periodic pore structure 17 b, said thirdperiodic pore structure having a third period length d_(S3) that issmaller than the second period length d_(S2) and serving for reducingthe adhesion of particles 17 c having a third particle diameter d_(p3)that is smaller than the second particle diameter d_(p2).

As was described further above, a surface structure 18 having a periodicpore structure having a predefined period length d_(S1) to d_(S3) cantypically only prevent the adhesion of particles 17 a-c whose particlediameter d_(P1) to d_(P3) is of a predefined order of magnitude. Thesurface structure shown in FIG. 4 serves to prevent the adhesion ofparticles 17 a-c having particle diameters dpi to d_(P3) which are ofdifferent orders of magnitude. For this purpose, it is necessary thatthe period lengths d_(S1) to d_(S3) of the periodic pore structures 17a-c are not too close to one another. Therefore, the first period lengthd_(S1) should be of at least five times the magnitude of the secondperiod length d_(S2) and the second period length d_(S2) should be of atleast five times the magnitude of the third period length d_(S3). Thethird, i.e. smallest, period length d_(S3) defines the minimum particlediameter d_(p3) which can be prevented from adhesion by the surfacestructure 18. In the example shown, the third period length d_(S3) isless than e.g. 10 nm.

The surface structure 18 shown in FIG. 4 can be provided both at thesurface 14 a of an optical element 14 and at the surface 15 a of anon-optical component 15 of the EUV lithography system 1. The surfacestructure 18 shown in FIGS. 3A and 3B can, of course, also be providedat the surface 15 a of a non-optical component 15. The use of periodicor approximately periodic pore structures 23, 23 a-c has proved to beadvantageous since such structures can be applied with the aid ofstructuring methods in which the surface structure or the surfacestructures is or are formed by self-assembly.

By way of example for the case where the surface 15 a of the non-opticalcomponent 15 is formed from aluminium, the surface structure 18 shown inFIGS. 3a,b can be produced by anodic oxidation in aqueous electrolytes,as is described in the article—cited further above—by A. P. Li et al.“Hexagonal pore arrays with a 50-420 nm interpore distance formed byself-organisation in anodic alumina”, J. Appl. Phys, 84 (11), 6023(1998). In particular, in the case of the method described therein, theperiod length d_(S) of the periodic pore structure 23 or the diameterd_(V) of a respective pore-shaped depression 24 can be varied byvariation of the applied voltage within wide limits (from a fewnanometers to the micrometers range).

However, the surface structure 18 can also be realized with the aid oflithographic methods, i.e. by applying a light-sensitive coating to thesurface 9 a, 10 a, 13 a, 14 a, 15 a, exposing the light-sensitive layerfor the purpose of structuring the light-sensitive layer, removing thecoating in the non-structured regions, and etching the surface 9 a, 10a, 13 a, 14 a, 15 a for the purpose of producing the pore-shapeddepressions in the regions not protected by the structured coating. In asubsequent step, the structured coating serving as an etching mask isremoved completely from the surface 9 a, 10 a, 13 a, 14 a, 15 a havingthe desired surface structure 18.

In order to produce a surface structure 18 such as is illustrated inFIG. 4, a plurality of such lithographic structuring processes can beperformed successively. In order to produce very small structures of afew nanometers, for example the third periodic pore structure 23 chaving the third period length d_(S3) as shown in FIG. 4, a micellarapproach can be used for structuring, this approach being based on theself-assembly of block copolymers loaded with metal salts in conjunctionwith a subsequent lithography process, as is described in thearticle—described further above—“Nano-structured micropatterns bycombination of block copolymer self-assembly and UV photolithography”,in Nanotechnology 17, 5027 (2006).

To summarize, by providing a surface structure 18 at a surface 9 a, 10a, 13 a, 14 a, 15 a which is arranged in a vacuum environment 16 andwhich is therefore subjected to contaminating particles 17 whoseparticle diameters are generally not in the macroscopic range, it ispossible to achieve an effective reduction of the adhesion of theseparticles 17 to the surface 9 a, 10 a, 13 a, 14 a, 15 a. The particles17 that do not adhere to the surface 9 a, 10 a, 13 a, 14 a, 15 a can beremoved from the vacuum system, for example the EUV lithography system1, through an extraction by suction (vacuum pumps).

What is claimed is:
 1. An optical element, comprising: a substrate, anda multilayer coating applied to the substrate and configured to reflectextreme ultraviolet (EUV) radiation, and having a surface, wherein asurface structure at the surface of the multilayer coating reducesadhesion of contaminating particles, wherein the surface structure haspore-shaped depressions separated from one another by webs, and whereinthe pore-shaped depressions have respective diameters (d_(V)) which aresmaller than diameters (d_(P)) of the contaminating particles.
 2. Theoptical element according to claim 1, wherein the pore-shapeddepressions have a diameter (d_(v)) of less than 10 nm.
 3. The opticalelement according to claim 1, wherein web widths (B) of the surfacestructure are smaller than the respective diameters (d_(v)) of thepore-shaped depressions of the surface structure.
 4. The optical elementaccording to claim 1, wherein a depth (T) of a respective pore-shapeddepression is at least as large as half the diameter (d_(V)/2) of arespective one of the pore-shaped depressions.
 5. The optical elementaccording to claim 1, wherein the surface structure has at least oneperiodic pore structure.
 6. The optical element according to claim 5,wherein the at least one periodic pore structure has a period length(d_(S), d_(S3)) of less than 10 nm.
 7. The optical element according toclaim 5, wherein the surface structure has a first periodic porestructure having a first period length (d_(S1)) and a second periodicpore structure applied to the first periodic pore structure and having asecond period length (d_(S2)) which is smaller than the period length(d_(S1)) of the first periodic pore structure.
 8. The optical elementaccording to claim 7, wherein the first period length (d_(S1)) is atleast five times the second period length (d_(S2)).